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Why my results change when I use a finer mesh?

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Old   November 16, 2021, 03:10
Default Why my results change when I use a finer mesh?
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amirhossein taghizadeh
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hi.
I use flow-3d for simulating a surface vortex with an air core in a pipe-tank system. I have been successful simulating the vortex with 1cm mesh size, but I haven't been able to see the air core. Due to this problem I had to had to use finer mesh with size of 0.6cm.
When using finer mesh ,my streamlines of the near surface particles change dramatically in comparison with the streamlines I had when using coarser mesh with 1cm mesh cell size.
In coarser mesh my streamlines are strongly sucked and driven into my pipe but when i reduce my mesh cell to 0.6cm, with exactly the same setup my streamline suddenly get shorter and incomplete. It seems that after changing the mesh size the flow of water in to the pipe is not enough strong to create the enough suction for near surface particles of water and due to this there i see no air entrainment afterward.
I'll be so delightful if anyone can tell what is the cause of this problem I have and how I can solve it?
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Old   November 16, 2021, 06:59
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Filippo Maria Denaro
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The streamlines mean nothing, you have to check your set of computed variables. Maybe you have numerical oscillations in the solution and the onset of a numerical instability.
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Old   November 17, 2021, 08:09
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Quote:
Originally Posted by FMDenaro View Post
The streamlines mean nothing, you have to check your set of computed variables. Maybe you have numerical oscillations in the solution and the onset of a numerical instability.
what kind of numerical oscillations do you mean??
does these oscillations because of mesh?? my mesh quality is good actually
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Old   November 17, 2021, 13:30
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Grid independent study is must for numerical simulation to reduce the influence of grid size on the computational results
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Old   November 18, 2021, 05:15
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Originally Posted by Kummi View Post
Grid independent study is must for numerical simulation to reduce the influence of grid size on the computational results
thanks for your answer but can I ask how exactly I can do grid independent study? just with trying different mesh sizes with different patterns?
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Old   November 18, 2021, 09:35
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Try to fix the mesh pattern. Example: for airfoil and pipe, boundary layer mesh pattern gives better accuracy and in other cases, the significant regime should have high/appropriate dense mesh pattern.

So once if you fix the pattern, alter the grid size/number from coarse to fine mesh. Let's say velocity is the important parameter in your case. Plot the velocity for different mesh numbers. After complete analysis at a particular mesh size/number, you can find that the velocity doesn't change much. If that happens, it confirms the optimum mesh for your domain. From here, even if you increase the mesh numbers, the numerical results doesn't vary much at any cost. This is called grid independent study which is basically necessary for any numerical problems.
Thank you
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Old   November 20, 2021, 01:37
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Quote:
Originally Posted by Kummi View Post
Try to fix the mesh pattern. Example: for airfoil and pipe, boundary layer mesh pattern gives better accuracy and in other cases, the significant regime should have high/appropriate dense mesh pattern.

So once if you fix the pattern, alter the grid size/number from coarse to fine mesh. Let's say velocity is the important parameter in your case. Plot the velocity for different mesh numbers. After complete analysis at a particular mesh size/number, you can find that the velocity doesn't change much. If that happens, it confirms the optimum mesh for your domain. From here, even if you increase the mesh numbers, the numerical results doesn't vary much at any cost. This is called grid independent study which is basically necessary for any numerical problems.
Thank you
thank you for your clarification
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